1. Field of the Invention
The present invention generally relates to the field of semiconductor processing. More particularly, the present invention relates to methods and apparatus for thermally processing a semiconductor substrate.
2. Description of the Related Art
Rapid thermal processing (RTP) is a process for annealing substrates during semiconductor processing. During RTP, thermal radiation is generally used to rapidly heat a substrate in a controlled environment to a maximum temperature of up to about 1350° C. This maximum temperature is maintained for a specific amount of time ranging from less than one second to several minutes depending on the process. The substrate is then cooled to room temperature for further processing. High intensity tungsten halogen lamps are commonly used as the source of heat radiation. The substrate may be provided additional heat by a heated susceptor conductively coupled to the substrate.
The semiconductor fabrication process has several applications of RTP. Such applications include thermal oxidation, high temperature soak anneal, low temperature soak anneal, and spike anneal. In thermal oxidation, a substrate is heated in oxygen, ozone, or a combination of oxygen and hydrogen which causes silicon substrate to oxidize to form silicon oxide. In high temperature soak anneal, a substrate is exposed to different gas mixtures such as nitrogen, ammonia, or oxygen. Low temperature soak anneal is generally used to anneal substrate deposited with metal. Spike anneal is used when the substrate needs to be exposed to high temperature for a very short time. During a spike anneal, the substrate is rapidly heated to a maximum temperature sufficient to activate dopant and cooled rapidly to end the activation process prior to substantial diffusion of the dopant.
RTP usually requires a substantially uniform temperature profile across the substrate. In the state of the art process, the temperature uniformity may be improved by controlling heat sources, such as a laser, an array of lamps, configured to heat the substrate on the front side while a reflective surface on the back side reflects heat back to the substrate. Emissivity measurement and compensation methodology have been used to improve the temperature gradient across the substrate.
As the semiconductor industry develops, the requirement for temperature uniformity during a RTP also increases. In some processes, it is important to have substantially small temperature gradient from about 2 mm inside the edge of the substrate. Particularly, it may be necessary to heat a substrate at a temperature between about 200° C. to about 1350° C. with a temperature deviation of about 1° C. to 1.5° C. The state of the art RTP systems have difficulties to reach this kind of uniformity, especially near the edge of the substrate. In a RTP system, an edge ring is usually used to support a substrate near the periphery. The edge ring and the substrate overlap producing a complicated heating situation near the edge of the substrate. In one aspect, the substrate may have different thermal properties near the edge. This is mostly pronounced for a patterned substrate, or for a silicon-on isulator-(SOI) substrate. In another aspect, the substrate and the edge ring overlap near the edge, it is difficult to achieve uniform temperature profile near the edge by measuring and adjusting the temperature of the substrate alone. Depending on the edge ring's thermal properties relative to the substrate's thermal and optical properties, the temperature profile of a substrate is generally either edge high or edge low.
FIG. 1 schematically illustrates two types of common temperature profiles of a substrate processed in a RTP chamber. The vertical axis denotes measured temperatures on a substrate. The horizontal axis denotes the distance from the edge of the substrate. Profile 1 is an edge high profile where the edge of the substrate has the highest temperature measurement. Profile 1 is an edge low profile where the edge of the substrate has the lowest temperature measurement. It is difficult to remove temperature deviation near the edge of the substrate in the state of the art RTP systems.
Therefore, there is a need for apparatus and methods used in RTP for improved temperature uniformity.